1. Field of the Invention
This invention relates to a method of fluidized catalyst cracking with crystalline zeolitic catalysts. More particularly, it relates to an improved method for increasing the overall efficiency of a fluidized catalytic cracker by subjecting the separated or spent cracking catalyst, after reaction with hydrocarbons to stripping in a dense phase to remove volatile hydrocarbons followed by an additional period of time in the presence of steam at the same temperature to condition the catalyst so that coke yield is further reduced and so that upon oxidative regeneration and recycling to the reaction zone additional gaseous and liquid products are generated at the expense of heavier hydrocarbons.
2. Description of the Prior Art
Fluidized cracking of heavy petroleum fractions is one of the major refining methods to convert crude petroleum oil to useful products, such as fuels for internal combustion engines. In such fluidized catalytic cracking, (known popularly as "FCC") high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely divided solid catalyst particles in an elongated riser or transfer line reactor. The transfer line is usually in the form of a riser tube and the contacting time is on the order of a few seconds, say from 1 to 10 seconds, and generally not over about 5 seconds. During this short period, catalysts at temperatures in the range of from about 1100.degree. F. to 1400.degree. F. are contacted with a hydrocarbon feedstock frequently in the form of vacuum gas oil, cycle oil or the like, heated to a temperature of from about 300.degree. to 800.degree. F. The reaction is one of essentially instantaneous generation of large volumes of gaseous hydrocarbons. The hydrocarbons and catalyst mixture flows out of the riser tube into a reactor chamber wherein the resultant gaseous hydrocarbons are taken off for distillation into various product fractions defined by boiling ranges. The spent catalyst is then separated in the reactor vessel and stripped of hydrocarbons by passing the catalyst through a stripper section which includes steam flowing up through the down flowing catalyst usually in a period of 1-3 minutes. Catalyst is then returned to a regenerator where residual hydrocarbons, called "coke", on the spent catalyst are burned off by passing a stream of an oxygen-containing gas, such as air, or oxygen enriched air, through them until substantially all the carbon is burned from the particles. The heat generated in this regeneration step is used as a heat source to heat the catalyst and thus provides elevated temperatures needed for reaction with the incoming hydrocarbon feed. Regenerated hot catalyst is then recycled to the reactor riser tube.
A particular problem for this type of catalytic cracking process is to effectively "strip" the catalyst of hydrocarbons before it is recirculated to the regenerator. Prior to the present invention it has been believed that excessive stripper residence time did not benefit the process. That is, after interstitial strippable hydrocarbons were removed, no additional benefit would be noticed. Even absorbed hydrocarbons cannot be removed, according to experts in the field, Venuto and Habib who have reviewed the literature ("Fluid Catalytic Cracking with Zeolite Catalysts" by Paul B. Venuto and E. Thomas Habib, Jr. published by Marcel Dekber, Inc., New York and Basel, copyright 1979, page 15). Reduced coke levels are important to avoid overheating the system by exothermic oxidative removal of this coke from the catalyst. Upon regeneration, it is particularly important to remove this coke to low levels to regain the activity and selectivity of zeolitic catalysts. Catalysts for modern fluid catalytic cracking units typically contain such zeolites. Generally, zeolite catalysts are composed of a silica and/or alumina base and may include other refractory metal oxides such as magnesia or zirconia. More particularly they may contain combinations of silica and alumina containing from near zero to 50 wt % alumina, and crystaline alumino-silicates. In these catalysts it is particularly important to reduce coke content of deposits on the catalyst down to a range of about 0.1 weight percent of carbon.
It has been proposed to avoid overcracking and excessive coke formation on such catalysts by rapid disengagement of the cracking catalyst from the mixture of hydrocarbonaceous material and catalyst flowing out of the riser pipe. Such action is particularly important as pointed out in copending U.S. patent application Ser. Nos. 238,380, filed Feb. 26, 1981, 335,458, filed Dec. 29, 1981 (now U.S. Pat. No. 4,414,100 issued Nov. 8, 1983), and 503,781, filed June 14, 1983, assigned to the assignee of the present invention. In those applications it is taught that a cyclonic motion imparted to the mixture of catalyst and hydrocarbonaceous material leaving the riser reactor effectively removes catalyst more quickly from the reaction product and improves the overall efficiency of the fluid catalytic cracking process. In said applications, the disclosures of which are incorporated herein by reference, there is disclosed a novel method of separating the catalyst after reaction with the hydrocarbonaceous material and before the catalyst is stripped for return to a regenerator by using a baffle arrangement adjacent the outflow from the reactor pipe.